Choice of Correct Type of Motor

We will now consider the various types of motors that are available and relative merits of each.

THE SQUIRREL CAGE ROTOR MOTOR.

This is the most common of motors that is used in industry today. It has the advantage that it is relatively economical to make and can be used, in conjunction with the correct starter for most drive applications. Average figures for this type of motor are as follows: Starting current 700% Full load (Depending on size this can vary from 480% to 900%) Starting torque 200% Full load (Depending on size this can vary from 100% to 300%) Pull out torque 300% Full load Pull up torque 100% Full Load Slip 3-4% (Small motors may have 6%. The slip decreases with output) Full load efficiency 77 to 95% (This increases with output but decreases with increase in poles) Power factor at full load 0.8 to 0.92 (This increases with output but decreases with increase in poles) The above figures are average and can vary greatly depending on output and manufacturer.

All manufacturers should be able to supply accurate figures for the rating and speed selected. Either higher starting torque or lower starting current can be obtained by different designs of the squirrel cage rotor. These advantages can only be obtained with increase in cost and/or decrease in power factor or efficiency, and it should be realized that, generally speaking, a high starting torque means a corresponding higher starting current. Similarly a lower starting current results in a reduced starting torque.

Having said that, special motors are available with high resistance rotors and high reactance rotors which change either the starting torque or starting current. If these are required it is suggested that full details of the driven load be provided to Western Electric and we will advise what we can offer. Special design rotors are available to provide such items as 300-400% locked rotor torque with locked rotor currents of 500 to 750% full load.

Two speed arrangements can be supplied with either tapped winding or double wound motors. The tapped wound motors are usually supplied where one speed is half of the other.

Example 4/8 pole.

Double wound motors are usually supplied where the speeds are other than half of each other.

Example 4/6 pole.

The tapped wound motor is cheaper than the double wound but requires more sophisticated control gear for operation.

As a general rule two speed motors have lower starting torque than single speed motors and in most instances the starting current is also lower. However this will depend on the speeds and motor used.

If these are required please contact Western Electric with full details of ratings and speeds required.

WOUND ROTOR MOTOR.

This type of motor is generally only used where frequent starting, accelerating high inertia loads, inching, and or plugging is required. They can also be used for speed control for such items as hoist motions on cranes etc where control must be provided over the load during lowering. However, as solid state starters are now available which can also provide these features for squirrel cage motors, their use has diminished over recent years. They also have the disadvantage that they require regular maintenance on the slip rings and brushes which can be expensive. If you feel that you require one of these motors please contact Western Electric with full details of the driven equipment and we will make a recommendation.

SYNCHRONOUS MOTORS

These are used where constant speed or operation at synchronous speed is required. An additional advantage of this type of motor is its ability to be over excited and draw leading kVAR from the supply, which can be used for power factor correction, particularly if the motor is operating a constant load. They are generally only used for large drives in large plants. They do require additional equipment for operation and are more expensive than an equivalent rating in squirrel cage. It is now possible with solid state drives to use a squirrel cage motor in some instances. If you feel that you require one of these motors please contact Western Electric with full details of the driven equipment and we will make a recommendation.

ENCLOSURE

There are a wide range of enclosures available for motors which depend upon the area they are to be operated in.

For average industrial use, where the atmosphere is free from hazardous gas etc, we would recommend the use of the Totally Enclosed Fan Cooled motor. This will permit operation of the motor, without further modification, indoors or in the open where rain, moisture or dust is present. If the motor is to be used in an area where abrasive dust, corrosive fumes, explosive gas or liquid, grain dust or textile fluff is present special motors may be required depending on the area classification.

The choice of enclosure should not be treated lightly and must be given careful consideration as, if the motor enclosure is not selected correctly, the motor may fail or cause an explosion or fire. Please refer to our comments in “Electric Motors for Use in Hazardous areas.”

If you are not certain as to what enclosure is required please advise Western Electric full details of the application and we will make our recommendations. However, under current regulations the onus lies with the end user to make the correct selection and Western Electric can only offer suggestions based on the information given.

DUTY TIME RATED MOTORS

It is possible to supply squirrel cage motors in frame sizes that are different to the normal if they are duty time rated. If you refer to AS1359.30 it lists the following duty time rated classifications:

S1	Continuous running duty

S2	Short Time duty

S3	Intermittent periodic duty with insignificant starting time

S4	Intermittent periodic duty with significant starting time

S5	Intermittent periodic duty with electric braking

S6	Continuous operation duty.

S7	Continuous operation duty with electric braking

S8	Continuous operation duty with related load-speed changes

S9	Non-periodic Duty with load and speed changes.

If you have an application that requires a duty time rated motor please contact Western Electric with all relevant details and we will supply a motor that will suit your application.

STARTING CURRENT LIMITATIONS

Owing to the large currents drawn when motors are started direct on line, it is necessary for some limitations to be imposed to avoid disturbances to the supply system. These disturbances not only cause an annoying flicker to lights, and problems with TV sets, but in extreme cases may cause sensitive relays to operate, or cause disruption to essential machinery or medical equipment.

Therefore, all supply authorities restrict the starting currents to a maximum allowable level, and the restrictions of all the larger authorities are calculated on a similar basis. The maximum currents are stated in supply authority regulations. Some variations do exist in rural areas and in special circumstances. Please check with your supply authority and refer to the Australian standard.

The starting current limitation leads us to consider the next aspect on this blog.